Tidal currents ebb and flow with the tide, but their net effect over a full tidal cycle is not zero everywhere. This net displacement creates weak yet persistent tidal residual currents that, while much weaker than instantaneous tidal flows, can drive the long-term transport of water, nutrients, pollutants, and even red tides. A new high-resolution numerical study published in the Journal of Xiamen University (Natural Science) in May 2026 maps these currents from the Bohai Sea to the northern South China Sea, offering critical insights for coastal management and environmental protection.
Researchers from Xiamen University's College of Ocean and Earth Sciences and the 715th Research Institute of China State Shipbuilding Corporation Ltd. used the Regional Ocean Modeling System (ROMS) to simulate barotropic tidal motion across a vast domain spanning 99°E–150°E and 15°S–41°N. Their simulation, at a horizontal resolution of 0.05° with 50 vertical layers and 15 tidal constituents, is one of the most detailed to date for China's marginal seas. The study, available via DOI: 10.6043/j.issn.0438-0479.202412018, compares Eulerian residual currents (time-averaged velocity at fixed points) with Lagrangian residual currents (net displacement of water parcels) and tidal Stokes drift, revealing distinct regional patterns and the mechanisms that govern them.
The results show that in the Bohai Sea, a large anticyclonic circulation dominates, with velocities of 0.5–3 cm/s, except in the northern Bohai Strait where speeds can reach 4–10 cm/s. The Yellow Sea features several small cyclonic and anticyclonic eddies near the coast, while a southward residual current exits the Bohai Strait and flows along the central Yellow Sea. In the Taiwan Strait, residual currents flow predominantly northeastward, with a strong anticyclonic circulation around the Taiwan Bank. The study also finds that tidal Stokes drift is comparable in magnitude to Eulerian residual currents in shallow waters but negligible in deep waters. Consequently, Lagrangian residual currents in shallow regions are directed more strongly toward the coast and are slightly faster than their Eulerian counterparts, whereas the two are nearly identical in deep waters.
Bathymetric features such as coastlines, islands, shoals, and submarine ridges organize the residual-current field and generate numerous small-scale circulations. A residual-vorticity balance indicates that the interaction of bottom friction with velocity shear exerts the dominant control on the overall distribution of Eulerian residual currents. The bottom-friction term associated with water-depth gradients acts mainly in localized regions, while the Coriolis term influences the background residual vorticity and several regional structures.
These findings matter because tidal residual currents contribute significantly to the long-term transport and dispersion of pollutants, sediment, nutrients, and other suspended material. Previous estimates cited in the study indicate that tidal residual currents account for about 50–80% of the local flow between the Changjiang Estuary and the Subei Shoal and may become the dominant component in some shallow coastal areas. By clarifying where these currents are strongest and which mechanisms shape them, the study provides a physical basis for assessing long-term material transport across China's continental shelves. This can inform coastal environmental assessment, marine engineering, channel maintenance, and the sustainable use of coastal resources.
The research was supported by the National Natural Science Foundation of China (Grant No. 41776015) and the National Key Research and Development Program of China (Grant No. 2022YFF0801404). For more information, see the original source at http://dx.doi.org/10.6043/j.issn.0438-0479.202412018.
